Eur. Phys. J. B 79, 289-299 (2011)
https://doi.org/10.1140/epjb/e2010-10473-5

Physical properties of FeSe_0.5Te_0.5 single crystals grown under different conditions

¹ Experimental Physics 5, Center for Electronic Correlations and Magnetism, Institute of Physics, University of Augsburg, 86159 Augsburg, Germany
² Institute of Applied Physics, Academy of Sciences of Moldova, MD, 2028 Chisinau, Republic of Moldova
³ Experimental Physics 2, Institute of Physics, University of Augsburg, 86159 Augsburg, Germany

Corresponding author: ^a vladimir.tsurkan@physik.uni-augsburg.de

Received: 18 June 2010
Revised: 5 October 2010
Published online: 19 January 2011

Abstract

We report on structural, magnetic, conductivity, and thermodynamic studies of FeSe_0.5Te_0.5 single crystals grown by self-flux and Bridgman methods. The lowest values of the susceptibility in the normal state, the highest transition temperature T_c of 14.4 K, and the largest heat-capacity anomaly at T_c were obtained for pure (oxygen-free) samples. The critical current density j_c of 8.6 × 10⁴  A/cm² (at 2 K) achieved in pure samples is attributed to intrinsic inhomogeneity due to disorder at the anion sites. The samples containing an impurity phase of Fe₃O₄ show increased j_c up to 2.3 × 10⁵  A/cm² due to additional pinning centers. The upper critical field of ~500 kOe is estimated from the resistivity study in magnetic fields parallel to the c-axis using a criterion of a 50% drop of the normal state resistivity R_n. The anisotropy of the upper critical field γ_{H c2} = H^ab_c2/H_c2^c reaches a value ~6 at . Extremely low values of the residual Sommerfeld coefficient of about 1 mJ/mol K², compared to the normal state Sommerfeld coefficient γ_n = 25  mJ/mol K² for pure samples indicate a high volume fraction of the superconducting phase (up to 97%). The electronic contribution to the specific heat in the superconducting state is well described within a single-band BCS model with a temperature dependent gap   Δ(0 K) = 27(1) K. A broad cusp-like anomaly in the electronic specific heat observed at low temperatures in samples with suppressed bulk superconductivity is ascribed to a splitting of the ground state of the Fe²⁺ ions at the 2c sites. This contribution is fully suppressed in the ordered state in samples with bulk superconductivity.